Neuron-specific enolase (NSE) is a neuronal form of the glycolytic enzyme enolase, which was first found in extracts of brain tissue, and was later shown to be present in APUD (amine precursor uptake and decarboxylation) cells and neurons of the diffuse neuroendocrine system but not in other peripheral cells. 90 neuroendocrine neoplasias (APUDomas) (including islet-cell tumours, phaeochromocytomas, medullary thyroid carcinomas, oat-cell tumours, and APUDomas of the gut, pancreas, and lung) reacted strongly with antisera to NSE. In addition, large amounts of the enzyme were found by radioimmunoassay in the tumours (mean 1626 +/- 479 SEM ng of NSE/mg protein), whereas control non-endocrine neoplasias contained less than 15 ng NSE/mg protein. Thus NSE, a specific enzyme produced in the neural and endocrine systems, was found to be produced in considerable quantities by all types of APUDomas but not in any non-endocrine tumours. NSE seems to be a useful and easily detected marker which may be used to distinguish endocrine from nonendocrine neoplasias. Clinical detection of endocrine tumours is difficult and such tumours are often missed. Use of NSE as a marker may avoid this.